In conventional methods of manufacture, a camshaft is a complex piece of metal defining a shaft portion integral with cams, journals and a gear, and is made by casting or forging to the desired configuration and dimensions. Such camshaft takes a long time to make and, therefore, entails high manufacturing costs; even when it is successfully forged or cast and machined, it has the drawback of being more liable to breakage due to internal flaws.
To overcome these drawbacks, the present invention proposes to build a camshaft with pre-machined parts of simple configurations, namely, individual cam parts, journal parts, a gear part and a shaft part, the four principal components of a camshaft. According to the proposed method, each camshaft part may be made from any desired material specifically tailored to its specific duty, and can be accurately assembled to constitute a build-up camshaft with its cams and journals occupying the exact angular and axial positons dictated by the design of the mechanism associated with it in service. Build-up camshafts manufactured according to this invention have been employed in practical engines for driving the valve mechanism thereof, resulting in a finding that these camshafts are not only satisfactory functionally but also endowed with the additional advantage that the profile surfaces of cams, as well as the riding faces of tappets, are less liable to wear, and the occurrence of valve mechanism trouble is minimized.
The high resistance of the cams and gear to wear is accounted for by the fact that, because cam parts and the gear according to the proposed method are originally individually machined parts made from annular or ring-shaped workpieces of a different material than the shaft, the grain structure of cam and gear surfaces can be more refined and, since the workpieces are smaller in bulk size than otherwise, they lend themselves to more effective surface treatments, such as hardening.
The proposed method of assembly is not based on the press-fitting or shrink-fitting of cams to a shaft part to introduce some interference in the fit, which presents considerable difficulty of accurately indexing the cams to their exact angular positions at the time of fitting the cams to the shaft and, therefore, requires the use of an indexing device, which is necessarily complicated because of the high degree of accuracy demanded in positioning the cams on the shaft. In some prior instances of built-up camshaft manufacture where the cams were of steel, press-fitted to the shaft, and were additionally secured to the shaft by welding, the subsequent hardening by high-frequency induction heating caused weld beads to develop cracks during rapid chilling due to the stresses in that portion of metal in the cam affected by the interference fit. Such cracks, however, can be avoided by eliminating the interference fit.
Accordingly, it is the object of this invention to provide an improved assembly type crankshaft and method of manufacturing same.